CMOS or CCD image sensors are of interest in a wide variety of sensing and imaging applications in a wide range of fields including consumer, commercial, industrial, and space electronics. Imagers based on charge coupled devices (CCDs) are currently the most widely utilized. CCDs are employed either in front or back illuminated configurations. Front illuminated CCD imagers are more cost effective to manufacture than back illuminated CCD imagers such that front illuminated devices dominate the consumer imaging market. Front-illuminated imagers, however, have significant performance limitations such as low fill factor/low sensitivity. The problem of low fill factor/low sensitivity is typically due to shadowing caused by the presence of opaque metal bus lines, and absorption by an array circuitry structure formed on the front surface in the pixel region of a front-illuminated imager. Thus, the active region of a pixel is typically relatively small (low fill factor) in large format (high-resolution) front-illuminated imagers.
An effect called smear can occur when light incident on the front-side of a front-illuminated imager falls on its charge transfer region regions, resulting in unwanted generation of charge carriers that produce a noise signal in a resulting image. Referring now to FIG. 1, a technique for reducing or eliminating smear in a typical front-illuminated imager is depicted. The front-illuminated imager 2 includes a silicon substrate 4, with overlying and alternating light detection regions 6 and charge transfer regions 8, separated by pixel isolation implants 10. A CCD gate contact 12 overlies each of the charge transfer regions 8. To reduce smear, the charge transfer regions are protected from incident light by placing metal shields 14 over the CCD gate contacts 12 and the charge transfer regions 8. Light falling on the CCD regions can also be made to converge on the detection regions 6 by employing micro lens (not shown).
Back-illuminated semiconductor imaging devices are advantageous over front-illuminated imagers for high fill factor, better overall efficiency of charge carrier generation and collection, and are suitable for small pixel arrays. One goal of the performance of back illuminated, semiconductor imaging devices is that charge carriers generated by light incident on the back side should be driven to the front side quickly to avoid any horizontal drift, which may smear the image.
Accordingly, what would be desirable, but has not yet been provided, is a device and method for fabricating back illuminated imagers which incorporate some form of light shielding is needed to avoid smear. In the case of back-illuminated imagers, any such shielding should be underneath the charge transfer regions and also should be well aligned to them.